A subscriber access network comprising a plurality of switching nodes is commonly constructed from a plurality of subscriber terminal exchanges and, if necessary, transit exchanges which are interconnected via a No. 7 signaling system. Each of the terminals connected to the subscriber access network is assigned a public number of the numbering area in which the subscriber access network is located.
The invention is based on a routing scheme for such a subscriber access network as is described in
U.S. Pat. No. 5,375,167. The subscriber access network is constructed from a plurality of subscriber terminal exchanges and one or more higher-ranking transit exchanges. Each terminal exchange is assigned at least one block of numbers within the numbering range of the subscriber access network. The assignment of the blocks of numbers is managed by the higher-ranking transit exchange. Blocks of numbers are requested by one of the subordinate exchanges and assigned by the higher-ranking exchange. By adapting the routing tables of the terminal exchanges, it is possible to assign the public numbers to subscriber lines within the blocks of numbers assigned to a terminal exchange in an arbitrary manner.
For the establishment of a connection, the called public number entered in a connection request is analyzed successively as usual: The front part of the number addresses a block of numbers, and thus one of the subordinate exchanges. This part is analyzed by the higher-ranking exchange. The rear part of the number addresses a subscriber within the internal numbering range of a terminal exchange and is analyzed by the latter.
One disadvantage of such a subscriber access network is that it involves a considerable amount of technical complexity: Several terminal exchanges and at least one transit exchange are needed.
Particularly for an operator of a small subscriber access network who shares a numbering range with other operators and their subscriber access networks, this complexity is disproportionately high.
Another disadvantage for such an operator is that with this architecture, number portability between subscriber access networks sharing the same numbering range is difficult to implement.